This invention relates to pre-loading outer spools for multi-spool rotors. More specifically, the present invention relates to an apparatus and method of locking up a group of parts on a shaft in the absence of a tie-rod for doing so.
A multi-spool rotor can be a series of concentric shafts that can rotate independently of and be controlled independently of one another. Typically, a wheel or other element is attached to the shaft. Reference to a xe2x80x9cwheelxe2x80x9d is used herein to denote a rotating element on a shaft, such as a turbine, a compressor, or a fan. Reference herein to xe2x80x9clocking-upxe2x80x9d or xe2x80x9clock-upxe2x80x9d is intended to refer to a wheel on a shaft being positionally fixed relative to such shaft so that the radially, axially and/or tangentially directed movement of the wheel is prevented.
A typical approach to locking-up the rotating element on a hollow shaft is to apply a compressive load to the element on the end of the shaft by the use of a threaded rod, or tie-bolt, running through the middle of the shaft. Nuts are then tightened on both ends of the tie-bolt. FIG. 1 shows a prior art tie-bolt shaft lockup apparatus. In this prior apparatus, a tie-bolt 101 extends down the center of a hollow shaft 102. Threaded nuts 103, 104 at the threaded ends 105, 106 of the tie-bolt 101 are tightened by applying torque to the threaded nuts 103, 104. This arrangement completes the xe2x80x9clock-up loads loopxe2x80x9d where the hollow shaft and elements are in compression and the tie-bolt is in tension.
However, in this prior apparatus, when a required compressive load is very large, a larger tie-bolt is needed. Also, since there is a limit to the elastic strength of the tie-bolt 101, wheels 107 on a large diameter shaft become harder to lock-up without using an excessively large tie bolt 101. A practical limitation is therefore present on how much compressive force can be achieved. A more important limitation, perhaps, is that the center of the hollow shaft is taken up by the tie-bolt. Consequently, no additional independently rotating shaft can be placed within the space occupied by the tie-bolt.
In related art, Smith (U.S. Pat. No. 3,758,179) shows the use of Belleville disk springs for applying pressure axially to an externally mounted axial mechanical shaft seal. However, the Belleville disk springs are not used for locking-up any items on the shaft, as the seal is placed for the purpose of sealing a rotating shaft. Mulders (U.S. Pat. No. 4,363,608) uses a Belleville disk spring to pre-load an upper bearing in a thrust bearing assembly. In an example given, the Belleville disk spring supplies a positive load of 13,000 pounds. Sutton et al. (U.S. Pat. No. 4,865,529) uses an axial spring to preload a thrust bearing pivot at start-up and shut down of a turbo-pump, which becomes disengaged at high speeds in favor of radial hydrostatic bearings. Kasabian (U.S. Pat. No. 3,807,815) uses Belleville disk springs to provide equal loading on multiple bearings by placing the Belleville disk springs so as to be compressed between the axle mounted bearings. Hansson (U.S. Pat. No. 3,804,562) uses Belleville disk springs to provide axial loading to a rotor. Wilkinson et al. (U.S. Pat. No. 3,909,085) shows the use of either a Belleville disk spring or a corrugated (xe2x80x9cwavyxe2x80x9d) washer for spring loading axially an outer race for ball bearings. None of the above show the use of Belleville disk springs or wavy washers for locking up wheels on a shaft.
Notwithstanding the design barriers, multiple concentric shafts and wheels are useful for overall enhancement of air movement for air cycle machines. Two or more air cycle machines may be used to condition air in an aircraft. In such air cycle machinery, a single shaft may connect two or more wheels such as a turbine, fan or compressor. However, if the air cycle machines are placed in a serial arrangement, the space requirements for the machinery tends to increase linearly. On the other hand, if the air cycle machines can be placed in a concentric configuration, then space savings can be realized. Moreover, the number of part counts will be vastly reduced.
There is a need for an improved apparatus and method of locking up wheels on a shaft. Furthermore, a lock-up apparatus and method for concentric shafts having multiple wheels is not available at this time. Yet another need is for an apparatus and method that eliminates the need for a tie-bolt on at least one outer concentric shaft. Still another need is for an apparatus and method of locking up wheels on a shaft that provides simultaneous locking in the axial, radial and tangential directions.
The present invention provides an apparatus and method of axially preloading wheels or elements on a hollow shaft by compressive means and without the use of a tie-rod. The invention herein is a method and apparatus for providing pre-load to outer spools of multi-spool rotors, which utilizes only compression members to supply the pre-loads. This invention allows outer spools to be on a concentric hollow shaft.
In one aspect of the present invention, the compression members in the present invention are Belleville disk springs or wavy (corrugated) washers, both of which function as springs. The reference to xe2x80x9cBelleville disk springsxe2x80x9d refer to those springs manufactured by Associated Spring Corporation and others under the tradename Belleville Spring Washer(trademark). A xe2x80x9cwavy washerxe2x80x9d is used herein to refer to a wavy or corrugated washer, such as that manufactured by Smalley Steel Ring Company under the tradename Spiralwave(trademark) Wave Spring. Wavy washers are generally preferred over Belleville springs in that, for larger preloading requirements, they take up less axial space than the heavy-duty conical Belleville washers.
In another aspect of the present invention, a threaded nut on each side of the wheel assembly to be locked-up is disposed so that when tightened the threaded nut compresses a Belleville disk spring or wavy washer. The disk spring or wavy washer, in turn, transmits its spring loaded force to a first clamping ring which is disposed adjacent to the threaded nut. The first clamping ring locks-up the wheel assembly and prevents the spring or washer from extending/unraveling in the radial direction. On the other side of the wheel being locked-up, and away from the first clamping ring, is disposed a second clamping ring. A thrust bearing, for example, is disposed next to the second clamping ring. On the side of the thrust bearing away from the second clamping ring, a symmetrically related wheel assembly is disposed and comprises a third clamping ring, a second wheel being locked-up, a fourth clamping ring, a second Belleville disk spring or wavy washer, and a second threaded nut.
Another feature of the present invention is the use of an axially cut raceway (lower), or ball bearing keyway, and an axially cut raceway (upper), or ball bearing keyway, to hold ball bearings between the outer hollow shaft and the wheels attached to the outer shaft. The ball bearings replace a key and a key-way which would otherwise act to prevent transverse relative motion of the wheel and the shaft, i.e., rotational movement of the wheel about the shaft. The raceways, or ball keyways, can be milled to any required precision. The ball bearings, having no edges and corners, are less subject to breakage than the more typical rectangular solid keys. The ball bearings are more easily placed into operative position and with less need for a larger key-way (raceway). This is because the ball bearings roll in and are not subject to the same degree of sliding friction that a rectangular solid key would encounter. The ball keyways and the ball bearings act to prevent the relative motion between the wheel and the shaft in a tangential direction, i.e., tangential to the longitudinal axis of the shaft.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.